Mechanisms of hypoxic cell injury. Summary of the symposium presented at the 1990 annual meeting of the Society of Toxicology

Cytotoxic effects of thioxanthone derivatives as photoinitiators on isolated rat hepatocytes

Thioxanthone and its analogues, 2- or 4-isopropylthioxanthone, 2-chlorothioxanthone, 2,4-diethylthioxanthone (DETX) and xanthone, are used as photoinitiators of ultraviolet (UV) light-initiated curable inks. As these photoinitiators were found in numerous food/beverage products packaged in cartons printed with UV-cured inks, the cytotoxic effects and mechanisms of these compounds were studied in freshly isolated rat hepatocytes. The toxicity of DETX was greater than that of other compounds. DETX elicited not only concentration (0-2.0 mm)- and time (0-3 hours)-dependent cell death accompanied by the depletion of cellular adenosine triphosphate (ATP), and reduced glutathione (GSH) and protein thiol levels, but also the accumulation of GSH disulfide and malondialdehyde. Pretreatment of hepatocytes with either fructose at a concentration of 10 mm or N-acetyl-l-cysteine (NAC) at a concentration of 5.0 mm ameliorated DETX (1 mm)-induced cytotoxicity. Further, the exposure of hepatocytes to DETX resulted in the induction of reactive oxygen species (ROS) and loss of mitochondrial membrane potential, both of which were partially prevented by the addition of NAC. These results indicate that: (1) DETX-induced cytotoxicity is linked to mitochondrial failure and depletion of cellular GSH; (2) insufficient cellular ATP levels derived from mitochondrial dysfunction were, at least in part, ameliorated by the addition of fructose; and (3) GSH loss and/or ROS formation was prevented by NAC. Taken collectively, these results suggest that the onset of toxic effects caused by DETX may be partially attributable to cellular energy stress as well as oxidative stress.

Effect of the combined administration of vitamin-E and 5-aminosalicylic acid on acrylamide-induced testicular toxicity

OBJECTIVES

This study aimed to evaluate the comparative protective antioxidant effect of 5-aminosalicylic acid (5-ASA) and vitamin-E against acrylamide (ACR)-induced testicular toxicity in rats.

METHODS

This study was performed at King Fahad Medical Research Centre, Jeddah, KSA. A total of 49 adult Wistar rats (250 ± 20 gm) that were 60 days old were divided into seven groups (control, ACR alone, ACR + 5-ASA, ACR + Vitamin-E, ACR + 5-ASA + Vitamin-E, Vitamin-E alone, 5-ASA alone). Acrylamide [45 mg/kg (bw)/day] and vitamin-E [200 mg/kg (bw)/day] were gavaged orally, and 5-ASA [25 mg/kg (bw)/day] were injected intra-peritoneally for five consecutive days after one day of observation. Rats were sacrificed by cervical dislocation. Histopathology of the testis, enzyme linked immunosorbent assay (ELISA) of testosterone, the lactate dehydrogenase (LDH) assay and a caudal sperm count were performed.

RESULTS

Rats treated with ACR showed signs of aggression and rough coats, with reduced food and water intake. ACR treated rats showed histopathological changes in the form of a sloughed seminiferous epithelium in the tubular lumen with no multinucleated giant cells. Shrinkage of seminiferous tubules with widening of the interstitial space was also observed with atrophy and the shedding of normal mucosa. Our results indicated that maximum protection was conveyed by the combined antioxidant effect of vitamin-E and 5-ASA on testicular histopathology.

CONCLUSION

We conclude that acrylamide-induced degeneration of seminiferous tubules can be partially reversed by the administration of 5-ASA and vitamin-E and suggests restricting exposure to ACR.

Preventive effects of fructose and N-acetyl-L-cysteine against cytotoxicity induced by the psychoactive compounds N-methyl-5-(2-aminopropyl)benzofuran and 3,4-methylenedioxy-N-methamphetamine in isolated rat hepatocytes

Psychoactive compounds, N-methyl-5-(2-aminopropyl)benzofuran (5-MAPB) and 3,4-methylenedioxy-N-methamphetamine (MDMA), are known to be hepatotoxic in humans and/or experimental animals. As previous studies suggested that these compounds elicited cytotoxicity via mitochondrial dysfunction and/or oxidative stress in rat hepatocytes, the protective effects of fructose and N-acetyl-l-cysteine (NAC) on 5-MAPB- and MDMA-induced toxicity were studied in rat hepatocytes. These drugs caused not only concentration-dependent (0-4 mm) and time-dependent (0-3 hours) cell death accompanied by the depletion of cellular levels of adenosine triphosphate (ATP) and glutathione (reduced form; GSH) but also an increase in the oxidized form of GSH. The toxic effects of 5-MAPB were greater than those of MDMA. Pretreatment of hepatocytes with either fructose at a concentration of 10 mm or NAC at a concentration of 2.5 mm prevented 5-MAPB-/MDMA-induced cytotoxicity. In addition, the exposure of hepatocytes to 5-MAPB/MDMA caused the loss of mitochondrial membrane potential, although the preventive effect of fructose was weaker than that of NAC. These results suggest that: (1) 5-MAPB-/MDMA-induced cytotoxicity is linked to mitochondrial failure and depletion of cellular GSH; (2) insufficient cellular ATP levels derived from mitochondrial dysfunction were ameliorated, at least in part, by the addition of fructose; and (3) GSH loss via oxidative stress was prevented by NAC. Taken collectively, these results indicate that the onset of toxic effects caused by 5-MAPB/MDMA may be partially attributable to cellular energy stress as well as oxidative stress.

Tetrabromobisphenol A induces cellular damages in pancreatic β-cells in vitro

Tetrabromobisphenol A (TBBPA) is a well-known organobrominated flame retardant. TBBPA has been detected in the environment. The roles played by environmental pollutants in increasing the prevalence of metabolic syndrome are attracting increasing concern. In the present work, we investigated the effects of TBBPA on rat pancreatic β-cells (the RIN-m5F cell line). RIN-m5F cells were incubated with different concentrations of TBBPA for 48 h, and cell viability and the extent of apoptosis were determined. We also measured the levels of inflammatory cytokines, reactive oxygen species (ROS), mitochondrial adenosine triphosphate (ATP), and cardiolipin, as well as the extent of cytochrome c release from mitochondria. TBBPA reduced the ATP level, induced cardiolipin peroxidation and cytochrome c release, and triggered apoptotic cell death. Moreover, TBBPA increased the levels of inflammatory cytokines (TNF-α and IL-1β), nitric oxide, intracellular ROS, and mitochondrial superoxide. Together, our results indicate that TBBPA damages pancreatic β-cells by triggering mitochondrial dysfunction and inducing apoptosis.

Mitochondria-meditated pathways of organ failure upon inflammation

Liver failure induced by systemic inflammatory response (SIRS) is often associated with mitochondrial dysfunction but the mechanism linking SIRS and mitochondria-mediated liver failure is still a matter of discussion. Current hypotheses suggest that causative events could be a drop in ATP synthesis, opening of mitochondrial permeability transition pore, specific changes in mitochondrial morphology, impaired Ca²⁺ uptake, generation of mitochondrial reactive oxygen species (mtROS), turnover of mitochondria and imbalance in electron supply to the respiratory chain. The aim of this review is to critically analyze existing hypotheses, in order to highlight the most promising research lines helping to prevent liver failure induced by SIRS. Evaluation of the literature shows that there is no consistent support that impaired Ca⁺⁺ metabolism, electron transport chain function and ultrastructure of mitochondria substantially contribute to liver failure. Moreover, our analysis suggests that the drop in ATP levels has protective rather than a deleterious character. Recent data suggest that the most critical mitochondrial event occurring upon SIRS is the release of mtROS in cytoplasm, which can activate two specific intracellular signaling cascades. The first is the mtROS-mediated activation of NADPH-oxidase in liver macrophages and endothelial cells; the second is the acceleration of the expression of inflammatory genes in hepatocytes. The signaling action of mtROS is strictly controlled in mitochondria at three points, (i) at the site of ROS generation at complex I, (ii) the site of mtROS release in cytoplasm via permeability transition pore, and (iii) interaction with specific kinases in cytoplasm. The systems controlling mtROS-signaling include pro- and anti-inflammatory mediators, nitric oxide, Ca²⁺ and NADPH-oxidase. Analysis of the literature suggests that further research should be focused on the impact of mtROS on organ failure induced by inflammation and simultaneously providing a new theoretical basis for a targeted therapy of overwhelmed inflammatory response.

•

Relationship between mitochondrial dysfunction and high lethality upon sepsis.

•

Criteria to define critical for lethality mitochondrial dysfunction.

•

ATP, calcium, mitochondrial ultrastructure and apoptosis, upon inflammation.

•

Regulation of inflammatory processes by mitochondrial ROS.

Antihyperlipidemic and antiapoptotic potential of zingerone on alcohol induced hepatotoxicity in experimental rats

The ultimate aim of this present study was to investigate the antihyperlipidemic and antiapoptotic potential of zingerone (ZO) on alcohol induced hepatotoxicity in experimental rats. Male albino wistar rats were divided in four groups. Groups 1 and 2 rats received isocaloric glucose and dimethyl sulphoxide (2% DMSO), liver toxicity was induced in groups 3 and 4 by supplementing 30% ethanol post orally for 60 days. In addition to, groups 2 and 4 received zingerone (20 mg/kg body weight in 2% DMSO) daily during the final 30 days of the experimental period. Ethanol alone administered rats showed increased levels/activities of plasma total cholesterol (TC), triglycerides (TG), free fatty acids (FFA), phospholipids (PL), low density lipoproteins (LDL), very low density lipoproteins (VLDL), tissue TC, TG, FFA, PL, HMG-CoA reductase, phase I xenobiotic enzymes, collagen and fat accumulation, DNA damage and increased Bax, caspase-3 and caspase-9 expressions and decrease in the levels/activities of plasma high density lipoproteins (HDL), lipoprotein lipase (LPL), lecithin cholesterol acyl transferase (LCAT), phase II xenobiotic enzymes and a decreased Bcl-2 expression. Zingerone supplementation was able to counter and reverse the ethanol induced changes in all the above parameters in experimental rats. Together results portray zingerone exhibits antihyperlipidemic and antiapoptotic potential on alcohol induced hepatotoxicity.

The impact of sterile inflammation in acute liver injury

BACKGROUND

The liver has a number of functions in innate immunity. These functions predispose the liver to innate immune-mediated liver injury when inflammation goes unchecked. Significant progress has been made in the last 25 years on sterile inflammatory liver injury in a number of models; however, a great deal of controversy and many questions about the nature of sterile inflammation still exist.

AIM

The goal of this article is to review sterile inflammatory liver injury using both a basic approach to what constitutes the inflammatory injury, and through examination of current models of liver injury and inflammation. This information will be tied to human patient conditions when appropriate.

RELEVANCE FOR PATIENTS

Inflammation is one of the most critical factors for managing in-patient liver disease in a number of scenarios. More information is needed for both scientists and clinicians to develop rational treatments.

Cytotoxic effects of psychotropic benzofuran derivatives, N-methyl-5-(2-aminopropyl)benzofuran and its N-demethylated derivative, on isolated rat hepatocytes

The novel psychoactive compounds derived from amphetamine have been illegally abused as recreational drugs, some of which are known to be hepatotoxic in humans and experimental animals. The cytotoxic effects and mechanisms of 5-(2-aminopropyl)benzofuran (5-APB) and N-methyl-5-(2-aminopropyl)benzofuran (5-MAPB), both of which are benzofuran analogues of amphetamine, and 3,4-methylenedioxy-N-methamphetamine (MDMA) were studied in freshly isolated rat hepatocytes. 5-MAPB caused not only concentration-dependent (0-4.0 mm) and time-dependent (0-3 h) cell death accompanied by the depletion of cellular ATP and reduced glutathione and protein thiol levels, but also accumulation of oxidized glutathione. Of the other analogues examined at a concentration of 4 mm, 5-MAPB/5-APB-induced cytotoxicity with the production of reactive oxygen species and loss of mitochondrial membrane potential was greater than that induced by MDMA. In isolated rat liver mitochondria, the benzofurans resulted in a greater increase in the rate of state 4 oxygen consumption than did MDMA, with a decrease in the rate of state 3 oxygen consumption. Furthermore, the benzofurans caused more of a rapid mitochondrial swelling dependent on the mitochondrial permeability transition than MDMA. 5-MAPB at a weakly toxic level (1 mm) was metabolized slowly: levels of 5-MAPB and 5-APB were approximately 0.9 mm and 50 μm, respectively, after 3 h incubation. Taken collectively, these results indicate that mitochondria are target organelles for the benzofuran analogues and MDMA, which elicit cytotoxicity through mitochondrial failure, and the onset of cytotoxicity may depend on the initial and/or residual concentrations of 5-MAPB rather than on those of its metabolite 5-APB. Copyright © 2016 John Wiley & Sons, Ltd.

Comparative effects of sulfhydryl compounds on target organellae, nuclei and mitochondria, of hydroxylated fullerene-induced cytotoxicity in isolated rat hepatocytes

DNA damage and cytotoxicity induced by a hydroxylated fullerene [C60 (OH)24 ], which is a spherical nanomaterial and/or a water-soluble fullerene derivative, and their protection by sulfhydryl compounds were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C60 (OH)24 at a concentration of 50 μM caused time (0 to 3 h)-dependent cell death accompanied by the formation of cell surface blebs, the loss of cellular levels of ATP and reduced glutathione, accumulation of glutathione disulfide, and induction of DNA fragmentation assayed using alkali single-cell agarose-gel electrophoresis. C60 (OH)24 -induced cytotoxicity was effectively prevented by pretreatment with sulfhydryl compounds. N-acetyl-L-cysteine (NAC), L-cysteine and L-methionine, at a concentration of 2.5 mM, ameliorated cell death, accompanied by a decrease in cellular ATP levels, formation of cell surface blebs, induction of reactive oxygen species (ROS) and loss of mitochondrial membrane potential caused by C60 (OH)24 . In addition, DNA fragmentation caused by C60 (OH)24 was also inhibited by NAC, whereas an antioxidant ascorbic acid did not affect C60 (OH)24 -induced cell death and DNA damage in rat hepatocytes. Taken collectively, these results indicate that incubation of rat hepatocytes with C60 (OH)24 elicits DNA damage, suggesting that nuclei as well as mitochondria are target sites of the hydroxylated fullerene; and induction of DNA damage and oxidative stress is ameliorated by an increase in cellular GSH levels, suggesting that the onset of toxic effects may be partially attributable to a thiol redox-state imbalance caused by C60 (OH)24 .

Effects of N-acetyl-L-cysteine on target sites of hydroxylated fullerene-induced cytotoxicity in isolated rat hepatocytes

The effects of N-acetyl-L-cysteine (NAC) on cytotoxicity caused by a hydroxylated fullerene [C60(OH)24], which is known a nanomaterial and/or a water-soluble fullerene derivative, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C60(OH)24 at a concentration of 0.1 mM caused time (0-3 h)-dependent cell death accompanied by the formation of cell blebs, loss of cellular ATP, and reduced glutathione (GSH) and protein thiol levels, as well as the accumulation of glutathione disulfide and malondialdehyde (MDA), indicating lipid peroxidation. Despite this, C60(OH)24-induced cytotoxicity was effectively prevented by NAC pretreatment ranging in concentrations from 1 to 5 mM. Further, the loss of mitochondrial membrane potential (MMP) and generation of oxygen radical species in hepatocytes incubated with C60(OH)24 were inhibited by pretreatment with NAC, which caused increases in cellular and/or mitochondrial levels of GSH, accompanied by increased levels of cysteine via enzymatic deacetylation of NAC. On the other hand, severe depletion of cellular GSH levels caused by diethyl maleate at a concentration of 1.25 mM led to the enhancement of C60(OH)24-induced cell death accompanied by a rapid loss of ATP. Taken collectively, these results indicate that pretreatment with NAC ameliorates (a) mitochondrial dysfunction linked to the depletion of ATP, MMP, and mitochondrial GSH level and (b) induction of oxidative stress assessed by reactive oxygen species generation, losses of intracellular GSH and protein thiol levels, and MDA formation caused by C60(OH)24, suggesting that the onset of toxic effects is at least partially attributable to a thiol redox-state imbalance as well as mitochondrial dysfunction related to oxidative phosphorylation.

Hepatoprotective effects of vitamin E/selenium against malathion-induced injuries on the antioxidant status and apoptosis-related gene expression in rats

The present study is undertaken to evaluate the protective effect of vitamin E (α-tocopherol) and selenium (Se) against malathion (MTN)-induced oxidative stress and hepatic injuries in experimental rats. Male rats were randomly divided into eight groups comprised of 10 rats each. The 1(st) group served as a negative control (C(N)), whereas the 2(nd) was supplemented with a combination of α-tocopherol (100 mg kg(-1) body weight, b.w.)/Se (0.1 mg kg(-1) bw). The 3(rd), 4(th) and 5(th) groups were respectively administered with increasing doses of MTN equivalent to (1/50 )LD(50) (M(1/50)), (1/25) LD(50) (M(1/25)) and (1/10) LD(50) (M(1/10)), respectively. The 6(th), 7(th) and 8(th) groups were administered the same doses of MTN as in the 3(rd), 4(th) and 5(th) groups with a concomitant supplementation with α-tocopherol/Se. Subchronic exposure of rats to MTN for 45 days resulted in statistical dose-dependent decrease in acetylcholinestrase (AChE) activity, increase in oxidative stress marker lipid peroxidation (LPO) and reduction in reduced glutathione (GSH) level. Moreover, the levels of glutathione persoxidase (GPx), superoxide dismutase (SOD) and catalase (CAT) were significantly decline in response to MTN exposure in a dose-dependent fashion. Furthermore, histopathological studies of liver in the rats which received MTN exhibited, moderate to severe degenerative and necrotic changes in the hepatocytes. Notably, the administration of α-tocopherol/Se protected the liver of rats exposed to MTN as evidenced by the appearance of normal histological structures, significant attenuation of the decline in all antioxidant enzymes tested (i.e. GPx, SOD and CAT), significant recovery in the GSH level and statistical reduction in LPO, as compared to the experimental rat. The effect of α-tocopherol/Se supplementation on transcriptional activity of three key stress and apoptosis-related genes (i.e., Tp53, CASP3 and CASP9), in response to MTN exposure in rats, was investigated. Results revealed a significant concentration-dependent up-regulation in the level of expression for the three genes examined, in response to MTN exposure, compared with the control. Interestingly, the supplementation of MTN-treated rats with α-tocopherol/Se modulates the observed significant dose-dependent up-regulation in the level of expression for three selected genes, indicative of an interfering role in the signaling transduction process of MTN-mediated poisoning. Taken together, these data suggest that the administration of α-tocopherol/Se may partially protect against MTN-induced hepatic oxidative stress and injuries.

Cytotoxic effects of hydroxylated fullerenes on isolated rat hepatocytes via mitochondrial dysfunction

The cytotoxic effects of hydroxylated fullerenes, also termed fullerenols or fullerols [C(60)(OH)( n )], which are known nanomaterials and water-soluble fullerene derivatives, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C(60)(OH)(24) caused not only concentration (0-0.25 mM)- and time (0-3 h)-dependent cell death accompanied by the formation of cell blebs, loss of cellular ATP, reduced glutathione (GSH), and protein thiol levels, but also the accumulation of glutathione disulfide and malondialdehyde, indicating lipid peroxidation. Of the other analogues examined, the cytotoxic effects of C(60)(OH)(12) and fullerene C(60) at a concentration of 0.125 mM were less than those of C(60)(OH)(24). The loss of mitochondrial membrane potential and generation of oxygen radical species in hepatocytes incubated with C(60)(OH)(24) were greater than those with C(60)(OH)(12) and fullerene C(60). In the oxygen consumption of mitochondria isolated from rat liver, the ratios of state-3/state-4 respiration were more markedly decreased by C(60)(OH)(24) and C(60)(OH)(12) compared with C(60). In addition, C(60)(OH)(24) and C(60)(OH)(12) resulted in the induction of the mitochondrial permeability transition (MPT), and the effects of C(60)(OH)(12) were less than those of C(60)(OH)(24). Taken collectively, these results indicate that (a) mitochondria are target organelles for fullerenols, which elicit cytotoxicity through mitochondrial failure related to the induction of the MPT, mitochondrial depolarization, and inhibition of ATP synthesis in the early stage and subsequently oxidation of GSH and protein thiols, and lipid peroxidation through oxidative stress at a later stage; and (b) the toxic effects of fullerenols may depend on the number of hydroxyl groups participating in fullerene in rat hepatocytes.

Perturbation in kidney lipid metabolic profiles in diabetic rats with reference to alcoholic oxidative stress

Diabetes is a major threat to global public health, and the number of diabetic patients is rapidly increasing worldwide. Evidence suggests that oxidative stress is involved in the pathophysiology of diabetic complications and alcoholic diseases. The aim of this study is to find out the impact of alcohol on lipid metabolic profiles in kidney tissue under streptozotocin induced diabetic condition. No study has been reported so far on the effect of alcohol on diabetic condition and also with reference to lipid metabolic profiles. Hence, the present study has been designed to elucidate the impact of alcoholism on diabetic condition. Male wistar strain albino rats were randomly divided into four groups: control (saline treated) NC, alcohol-treated (At), diabetic control (DC), and alcohol-treated diabetic rats (D+At). In alcohol-treated diabetic rats, we observed high levels of MDA, total cholesterol, triglycerides, phospholipids and also high levels of blood glucose than other groups. Moreover, degenerative changes of renal cells in alcohol-treated diabetic group were maximized by administration of alcohol as evinced by histopathological examination. This study suggests that alcohol consumption could be an aggravation factor which contributes for the formation of free radicals in diabetic condition. Therefore, consumption of alcohol during diabetic condition is harmful.

Cytotoxic effects of 3,4-methylenedioxy-N-alkylamphetamines, MDMA and its analogues, on isolated rat hepatocytes

The amphetamine-derived designer drugs have been illegally used worldwide as recreational drugs, some of which are known to be hepatotoxic in humans. To compare their cytotoxic effects, 3,4-methylenedioxy-N-methamphetamine (MDMA) and its related analogues, N-methyl-1-(3,4-methylenedioxyphenyl)-2-butanamine (MBDB), 3,4-(methylenedioxyphenyl)-2-butanamine (BDB) and 2-methylamino-1-(3,4-methylenedioxyphenyl)-propane-1-one (methylone) were studied in freshly isolated rat hepatocytes. MBDB caused not only concentration (0-4.0 mM)- and time (0-2 h)-dependent cell death accompanied by the formation of cell blebs, and the loss of cellular ATP and adenine nucleotide pools, and reduced glutathione levels, but also the accumulation of oxidized glutathione. Of the other analogues examined, the cytotoxicity of MBDB and BDB was greater than that of MDMA and methylone, suggesting that hepatotoxicity is generally induced by these drugs. In addition, DNA damage and the induction of reactive oxygen species were greater after the incubation of hepatocytes with MBDB (2 and 4 mM) than after that with MDMA. In isolated liver mitochondria, MBDB/BDB resulted in a greater increase in the rate of state 4 oxygen consumption than did MDMA/methylone, indicating an uncoupling effect and a decrease in the rate of state 3 oxygen consumption in a concentration dependent manner. Furthermore, MBDB resulted in mitochondrial swelling dependent on the mitochondrial permeability transition (MPT); the effect of MDMA was less than that of MBDB. Taken collectively, these results suggest that (1) the onset of cytotoxicity caused by designer drugs such as MBDB and MDMA is linked to mitochondrial failure dependent upon the induction of the MPT accompanied by mitochondrial depolarization and depletion of ATP through uncoupling of oxidative phosphorylation in rat hepatocytes, and (2) MBDB and MDMA elicit DNA damage, suggesting that nuclei as well as mitochondria are target sites of these compounds.

Biotransformation and cytotoxicity of a brominated flame retardant, tetrabromobisphenol A, and its analogues in rat hepatocytes

The metabolism and cytotoxic effects of tetrabromobisphenol A (TBBPA), a phenolic flame retardant, and its analogues were studied in freshly isolated rat hepatocytes and isolated hepatic mitochondria, respectively. The exposure of hepatocytes to TBBPA caused not only concentration (0.25-1.0 mM)- and time- (0-3 h) dependent cell death accompanied by the loss of cellular ATP, adenine nucleotide pools, reduced glutathione, and protein thiols, but also the accumulation of oxidized glutathione and malondialdehyde, indicating lipid peroxidation. TBBPA at a weakly toxic level (0.25 mM) was metabolized to monoglucuronide and monosulfate conjugates: the amounts of glucuronide rather than sulfate conjugate predominantly increased, accompanied by a loss of the parent compound, with time. In comparative effects based on cell viability, mitochondrial membrane potential and some toxic parameters, bisphenol A (BPA) was less toxic than TBBPA and tetrachlorobisphenol A (TCBPA), which are not significant differences in these parameters. In mitochondria isolated from rat liver, TBBPA and TCBPA caused an increase in the rate of State 4 oxygen consumption in the presence of succinate, indicating an uncoupling effect and a decrease in the rate of State 3 oxygen consumption in a concentration-dependent manner (5-25 microM). Taken collectively, our results indicate that (i) mitochondria are target organelles for TBBPA, which elicits cytotoxicity through mitochondrial dysfunction related to oxidative phosphorylation at an early stage and subsequently lipid peroxidation at a later stage; and (ii) the toxicity of TBBPA and TCBPA is greater than that of BPA, suggesting the participation of halogen atoms such as bromine and chlorine in the toxicity.

ATP-generating glycolytic substrates prevent N-nitrosofenfluramine-induced cytotoxicity in isolated rat hepatocytes

The relationship between cytotoxicity induced by N-nitrosofenfluramine and mitochondrial or glycolytic adenosine triphosphate (ATP) synthesis-dependent intracellular bioenergetics was studied in isolated rat hepatocytes. The supplementation of fructose, an ATP-generating glycolytic substrate, to hepatocyte suspensions prevented N-nitrosofenfluramine-induced cell injury accompanied by the formation of cell blebs, abrupt loss of intracellular ATP and reduced glutathione and mitochondrial membrane potential (DeltaPsi), and the accumulation of oxidized glutathione and malondialdehyde, indicating lipid peroxidation, during a 2h incubation period. Fructose (1-20mM) resulted in concentration-dependent protection against the cytotoxicity of N-nitrosofenfluramine at a concentration of 0.6mM, a low toxic dose. Pretreatment with xylitol, another glycolytic substrate, at concentration of 15mM also prevented the cytotoxicity caused by the nitroso compound, but neither glucose nor sucrose exhibited protective effects. In addition, fructose inhibited N-nitrosofenfluramine (0.5 and 0.6mM)-induced DNA damage, as evaluated in the comet assay, indicating that nuclei as well as mitochondria are target sites of the compound. These results indicate that (a) the onset of N-nitrosofenfluramine-induced cytotoxicity in rat hepatocytes is linked to mitochondrial failure, and that (b) the insufficient supply of ATP in turn limits the activities of all energy-requiring reactions and consequently leads to acute cell death.

Role of mitochondrial membrane permeability transition in N-nitrosofenfluramine-induced cell injury in rat hepatocytes

The role of mitochondrial membrane permeability transition in N-nitrosofenfluramine-induced cell injury was studied in mitochondria and hepatocytes isolated from rat liver. Mitochondrial permeability transition has been proposed as a common final pathway in acute cell death through mitochondrial dysfunction. In isolated mitochondria, N-nitrosofenfluramine (0.25 to 1.0 mM) in the presence of Ca(2+) (50 microM) elicited a concentration-dependent induction of mitochondrial swelling dependent on mitochondrial permeability transition and the release of cytochrome c, both of which were prevented by pretreatment with a specific inhibitor of mitochondrial permeability transition, cyclosporin A (0.2 microM). The effects of N-nitrosofenfluramine on mitochondria were more potent than those of fenfluramine, which is a sympathomimetic amine with anorectic action. The pretreatment of isolated hepatocytes with cyclosporin A (2 microM) partially but not completely prevented N-nitrosofenfluramine (0.6 mM; a low toxic dose)-induced cell death, loss of cellular ATP, formation of cell blebs and decrease in mitochondrial membrane potential. These results suggest that the onset of N-nitrosofenfluramine-induced cytotoxicity is linked to mitochondrial failure dependent upon induction of mitochondrial permeability transition accompanied by mitochondrial depolarization, the release of cytochrome c and depletion of intracellular ATP through uncoupling of oxidative phosphorylation.

Safety assessment of esters of p-hydroxybenzoic acid (parabens)

Parabens are widely used as preservatives in food, cosmetic and pharmaceutical products. Acute, subchronic, and chronic studies in rodents indicate that parabens are practically non-toxic. Parabens are rapidly absorbed, metabolized, and excreted. In individuals with normal skin, parabens are, for the most part, non-irritating and non-sensitizing. However, application of compounds containing parabens to damaged or broken skin has resulted in sensitization. Genotoxicity testing of parabens in a variety of in vitro and in vivo studies primarily gave negative results. The paraben structure is not indicative of carcinogenic potential, and experimental studies support these observations. Some animal studies have reported adverse reproductive effects of parabens. In an uterotrophic assay, methyl and butyl paraben administered orally to immature rats were inactive, while subcutaneous administration of butyl paraben produced a weak positive response. The ability of parabens to transactivate the estrogen receptor in vitro increases with alkyl group size. The detection of parabens in a small number of breast tumor tissue samples and adverse reproductive effects of parabens in animals has provoked controversy over the continued use of these substances. However, the possible estrogenic hazard of parabens on the basis of the available studies is equivocal, and fails to consider the metabolism and elimination rates of parabens, which are dose, route, and species dependent. In light of the recent controversy over the estrogenic potential of parabens, conduct of a reproductive toxicity study may be warranted.

N-Nitrosofenfluramine induces cytotoxicity via mitochondrial dysfunction and oxidative stress in isolated rat hepatocytes

The cytotoxic effects of fenfluramine, an appetite suppressant, and its N-nitroso derivative, N-nitrosofenfluramine, have been studied in freshly isolated rat hepatocytes and isolated hepatic mitochondria. Exposure of hepatocytes to N-nitrosofenfluramine caused not only concentration (0.25-1.0 mmol L(-1)) and time (0-3 h)-dependent cell death accompanied by the loss of cellular ATP, adenine nucleotide pools, reduced glutathione (GSH), and protein thiols, but also the accumulation of oxidized glutathione and malondialdehyde (MDA), indicating lipid peroxidation. There was a time lag for the onset of the accumulation of MDA after the rapid depletion of ATP. Supplementation of the hepatocyte suspensions with N-acetylcysteine (4 mmol L(-1)), a precursor of intracellular GSH, partially inhibited N-nitrosofenfluramine (1 mmol L(-1))-induced cytotoxicity. In comparative effects based on cell viability and rhodamine 123 retention, an index of mitochondrial membrane potential, fenfluramine was less toxic than N-nitrosofenfluramine. In mitochondria isolated from rat liver, N-nitrosofenfluramine caused an increase in the rate of state-4 oxygen consumption, indicating an uncoupling effect, and a decrease in the rate of state-3 oxygen consumption in a concentration-dependent manner. These results indicate that (a) mitochondria are target organelles for N-nitrosofenfluramine, which elicits cytotoxicity through mitochondrial dysfunction related to membrane potential and/or oxidative phosphorylation at an early stage and subsequently lipid peroxidation at a later stage; and (b) the toxicity of N-nitrosofenfluramine is greater than that of fenfluramine, suggesting participation of the nitroso group in the toxicity.

Chlorpropham induces mitochondrial dysfunction in rat hepatocytes

The metabolism and action of chlorpropham (isopropyl N-(3-chlorophenyl)carbamate; CIPC, a post-harvest agent) and its metabolites were studied in freshly isolated rat hepatocytes and isolated rat hepatic mitochondria, respectively. The exposure of hepatocytes to CIPC caused a concentration (0.25-1.0 mM)- and time (0-3h)-dependent cell death accompanied by loss of cellular ATP and adenine nucleotides. CIPC at a weakly toxic level (0.5 mM) was metabolized to isopropyl N-(3-chloro-4-hydroxyphenyl)carbamate (4OH-CIPC) and subsequently to its glucuronide and sulfate conjugates (major metabolites) or alternatively to a minor metabolite 3-chloroaniline (3CA). The addition of SKF-525A (50 microM), an inhibitor of microsomal monooxygenase, enhanced the CIPC (0.5 mM)-induced cytotoxicity accompanied by loss of ATP and 4OH-CIPC and inhibited the decrease in the concentration of the parent compound. CIPC led to a strong decrease in cellular ATP content compared to its metabolites, 4OH-CIPC and 3CA. On the other hand, the exposure of isolated hepatic mitochondria to CIPC reduced State 3 respiration with a FAD-linked substrate (succinate plus rotenone) and/or with a NAD+ -linked substrate (pyruvate plus malate), whereas State 3 respiration with ascorbate plus tetramethyl-p-phenylendiamine (cytochrome oxidase-linked respiration) was not affected markedly by CIPC. Further, the addition of CIPC caused an increase in the rate of State 4 oxygen consumption, indicating an uncoupling effect, and a decrease in the rate of State 3 oxygen consumption in a concentration-dependent manner, respectively. In contrast, the addition of neither 4OH-CIPC nor 3CA markedly affected the rate of states 3 and/or 4 oxygen consumption. These results indicate that CIPC-induced cytotoxicity is mediated by the parent compound rather than by its metabolites such as 4OH-CIPC and 3CA, and that the toxicity is associated with a rapid depletion of ATP via impairment of mitochondrial function related to oxidative phosphorylation.

Oxidative stress and loss of cortisol secretion in adrenocortical cells of rainbow trout (Oncorhynchus mykiss) exposed in vitro to endosulfan, an organochlorine pesticide

The effects of endosulfan, a widely used organochlorine pesticide, on cortisol secretion, cell viability, antioxidants and lipid peroxidation were investigated in enzymatically dispersed head kidney cells of rainbow trout (Oncorhynchus mykiss). ACTH- and dbcAMP-stimulated cortisol secretion, and cell viability were impaired in a dose-related manner following acute in vitro exposure to endosulfan (EC(50) 19 microM, LC(50) 366 microM) and the loss of cortisol secretion was detected even at concentrations of endosulfan that did not decrease cell viability. Stimulation with dbcAMP did not restore cortisol secretion in endosulfan exposed cells while stimulation with pregnenolone maintained cortisol secretion until viability of cells was affected. Thus endosulfan may disrupt processes between the step generating cAMP and the step where pregnenolone is used. Activity of catalase increased at concentrations of endosulfan that did not impair cortisol secretion, and decreased at higher doses. Glutathione peroxidase (GPx) activity was significantly reduced at doses of endosulfan that also reduced levels of glutathione, an essential cofactor of GPx. Exposure up to 1 x 10(-7) M endosulfan increased the activity of glutathione transferase. The present in vitro study identified endosulfan as a chemical inducing a loss of secretory responses in teleost adrenocortical steroidogenic cells and alterations in the activity of enzymes known to be involved in oxidative stress pathways. Moreover, the significant increase in lipid hydroperoxides levels provided further evidence for endosulfan-induced oxidative stress.

Effect of ischemic preconditioning on hepatic microcirculation and function in a rat model of ischemia reperfusion injury

Ischemic preconditioning (IPC) may protect the liver from ischemia reperfusion injury by nitric oxide formation. This study has investigated the effect of ischemic preconditioning on hepatic microcirculation (HM), and the relationship between nitric oxide metabolism and HM in preconditioning. Rats were allocated to 5 groups: 1. sham laparotomy; 2. 45 minutes lobar ischemia followed by 2-hour reperfusion (IR); 3. IPC with 5 minutes ischemia and 10 minutes reperfusion before IR; 4. L-arginine before IR; and 5. L-NAME + IPC before IR. HM was monitored by laser Doppler flowmeter. Liver transaminases, adenosine triphosphate, nitrites + nitrates, and guanosine 3'5'-cyclic monophosphate (cGMP) were measured. Nitric oxide synthase (NOS) distribution was studied using nicotinamide adeninine dinucleotide phosphate (NADPH) diaphorase histochemistry. At the end of reperfusion phase, in the IR group, flow in the HM recovered partially to 25.8% of baseline (P < .05 versus sham), whereas IPC improved HM to 49.5% of baseline (P < .01 versus IR). With L-arginine treatment, HM was 31.6% of baseline (NS versus IR), showing no attenuation of liver injury. In the preconditioned group treated with L-NAME, HM declined to 10.2% of baseline, suggesting not only a blockade of the preconditioning effect, but also an exacerbated liver injury. Hepatocellular injury was reduced by IPC, and L-arginine and was increased by NO inhibition with L-NAME. IPC also increased nitrate + nitrate (NOx) and cGMP concentrations. NOS detected by NADPH diaphorase staining was associated with hepatocytes and vascular endothelium, and was induced by IPC. IPC induced NOS and attenuated HM impairment and hepatocellular injury. These data strongly suggest a role for nitric oxide in IPC.

Safety assessment of propyl paraben: a review of the published literature

Propyl paraben (CAS no. 94-13-3) is a stable, non-volatile compound used as an antimicrobial preservative in foods, drugs and cosmetics for over 50 years. It is an ester of p-hydroxybenzoate. Propyl paraben is readily absorbed via the gastrointestinal tract and dermis. It is hydrolyzed to p-hydroxybenzoic acid, conjugated and the conjugates are rapidly excreted in the urine. There is no evidence of accumulation. Acute toxicity studies in animals indicate that propyl paraben is relatively non-toxic by both oral and parenteral routes, although it is mildly irritating to the skin. Following chronic administration, no-observed-effect levels (NOEL) as high as 1200-4000 mg/kg have been reported and a no-observed-adverse-effect level (NOAEL) in the rat of 5500 mg/kg is posited. Propyl paraben is not carcinogenic, mutagenic or clastogenic. It is not cytogenic in vitro in the absence of carboxyesterase inhibitors. The mechanism of propyl paraben may be linked to mitochondrial failure dependent on induction of membrane permeability transition accompanied by the mitochondrial depolarization and depletion of cellular ATP through uncoupling of oxidative phosphorylation. Sensitization has occurred when medications containing parabens have been applied to damaged or broken skin. Parabens have been implicated in numerous cases of contact sensitivity associated with cutaneous exposure, but high concentrations of 5-15% in patch testing are needed to elicit reaction in susceptible individuals. Allergic reactions to ingested parabens have been reported, although rigorous evidence of the allergenicity of ingested paraben is lacking.

Chloroacetonitrile (CAN) induces glutathione depletion and 8-hydroxylation of guanine bases in rat gastric mucosa

Chloroacetonitrile (CAN) is detected in drinking-water supplies as a by-product of the chlorination process. Gastroesophageal tissues are potential target sites of acute and chronic toxicity by haloacetonitriles (HAN). To examine the mechanism of CAN toxicity, we studied its effect on glutathione (GSH) homeostasis and its impact on oxidative DNA damage in gastric mucosal cells of rats. Following a single oral dose (38 or 76 mg/Kg) of CAN, animals were sacrificed at various times (0-24 h), and mucosa from pyloric stomach were collected. The effects of CAN treatment on gastric GSH contents and the integrity of genomic gastric DNA were assessed. Oxidative damage to gastric DNA was evaluated by measuring the levels of 8-Hydroxydeoxyguanosine (8-OHdG) in hydrolyzed DNA by HPLC-EC. The results indicate that CAN induced a significant, dose- and time-dependent, decrease in GSH levels in pyloric stomach mucosa at 2 and 4 hours after treatment (56 and 39% of control, respectively). DNA damage was observed electrophoretically at 6 and 12 hours following CAN administration. CAN (38 mg/Kg) induced significant elevation in levels of 8-OHdG in gastric DNA. Maximum levels of 8-OHdG in gastric DNA were observed at 6 hours after CAN treatment [9.59+/-0.60 (8-OHdG/10(5)dG) 146% of control]. When a high dose of CAN (76 mg/Kg) was used, a peak level of 8-OHdG [11.59+/-1.30 (8-OHdG/10(5)dG) 177% of control] was observed at earlier times (2 h) following treatment. When CAN was incubated with gastric mucosal cells, a concentration-dependent cyanide liberation and significant decrease in cellular ATP levels were detected. These data indicate that a mechanism for CAN-induced toxicity may be partially mediated by depletion of glutathione, release of cyanide, interruption of the energy metabolism, and induction of oxidative stress that leads to oxidative damage to gastric DNA.

Bioenergetic targeting during organ preservation: (31)P magnetic resonance spectroscopy investigations into the use of fructose to sustain hepatic ATP turnover during cold hypoxia in porcine livers

During liver preservation, ATP supplies become depleted, leading to loss of cellular homeostatic controls and a cascade of ensuing harmful changes. Anaerobic glycolysis is unable to prolong ATP production for a significant period because of metabolic blockade. Our aim was to promote glycolysis during liver cold hypoxia by supplying fructose as an additional substrate, compared to supplementation with an equivalent concentration of glucose. Porcine livers (two groups; n = 5 in each) were retrieved by clinical harvesting techniques and subjected to two cycles of cold hypoxia and oxygenated hypothermic reperfusion. In the second cycle of reperfusion, the perfusate was supplemented with either 10 mmol/L glucose (Group 1) or 10 mmol/L fructose (Group 2). During reperfusion in both groups, similar levels of ATP were detected by phosphorus magnetic resonance spectroscopy ((31)P MRS). However, during subsequent hypoxia, ATP was detected for much longer periods in the fructose-perfused group. The rate of ATP loss was sevenfold slower during hypoxia in the presence of fructose than in the presence of glucose (ATP consumption of -7.2 x 10(-3)% total (31)P for Group 1 versus -1.0 x 10(-3)% total (31)P for Group 2; P < 0. 001). The changes in ATP were mirrored by differences in other MRS-detectable intermediates; e.g., inorganic phosphate was significantly higher during subsequent hypoxia in Group 1 (45.7 +/- 2.7% total (31)P) than in Group 2 (33.7 +/- 1.1% total (31)P; P < 0. 01). High-resolution MRS of liver tissue extracts demonstrated that fructose was metabolized mainly via fructose 1-phosphate. We conclude that fructose supplied by brief hypothermic perfusion may improve the bioenergetic status of cold hypoxic livers by sustaining anaerobic glycolysis via a point of entry into the pathway that is different from that for glucose.

Metabolic depletion of ATP by fructose inversely controls CD95- and tumor necrosis factor receptor 1-mediated hepatic apoptosis

Hepatocyte apoptosis is crucial in several forms of liver disease. Here, we examined in different models of murine liver injury whether and how metabolically induced alterations of hepatocyte ATP levels control receptor-mediated apoptosis. ATP was depleted either in primary hepatocytes or in vivo by various phosphate-trapping carbohydrates such as fructose. After the activation of the tumor necrosis factor (TNF) receptor or CD95, the extent of hepatocyte apoptosis and liver damage was quantified. TNF-induced cell death was completely blocked in ATP-depleted hepatocyte cultures, whereas apoptosis mediated by CD95 was enhanced. Similarly, acute TNF-induced liver injury in mice was entirely inhibited by ATP depletion with ketohexoses, whereas CD95-mediated hepatotoxicity was enhanced. ATP depletion prevented mitochondrial cytochrome c release, loss of mitochondrial membrane potential, activation of type II caspases, DNA fragmentation, and cell lysis after exposure to TNF. The extent of apoptosis inhibition correlated with the severity of ATP depletion, and TNF-induced apoptosis was restored when ATP was repleted by increasing the extracellular phosphate concentration. Our study demonstrates that TNF-induced hepatic apoptosis can be selectively and reversibly blocked upstream of mitochondrial dysfunction by ketohexose-mediated ATP depletion.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Role of mitochondrial membrane permeability transition in p-hydroxybenzoate ester-induced cytotoxicity in rat hepatocytes

The relationship between mitochondrial membrane permeability transition (MPT) and the toxic effects of the alkyl esters of p-hydroxybenzoic acid (parabens) has been studied in mitochondria and hepatocytes isolated from rat liver. MPT has been proposed as a common final pathway in acute cell death through mitochondrial dysfunction. In isolated mitochondria, propyl-paraben (0.1 to 0.5 mM) in the presence of Ca2+ (50 microM) elicited a concentration-dependent induction of mitochondrial swelling dependent on MPT. This was prevented by pretreatment with a specific inhibitor of MPT, cyclosporin A (0.2 microM). For the other parabens tested, the induction of MPT depended on the relative elongation of alkyl side-chains in their molecular structure and was associated with the partition coefficients. In contrast, the induction caused by p-hydroxybenzoic acid was more potent than that of methyl- or ethyl-paraben. The pretreatment of freshly isolated hepatocytes with cyclosporin A (5 microM) and trifluoperazine (10 microM), which inhibit MPT in a synergistic manner, partially but not completely prevented propyl-paraben (1 mM; plus diazinon, 100 microM)-induced cell death, ATP loss, and decreased mitochondrial membrane potential. These results suggest that the onset of paraben-induced cytotoxicity is linked to mitochondrial failure dependent upon induction of MPT accompanied by the mitochondrial depolarization and depletion of cellular ATP through uncoupling of oxidative phosphorylation.

Alterations of cell volume regulation in the development of hepatocyte necrosis

Intracellular Na+ accumulation has been shown to contribute to hepatocyte death caused by anoxia or oxidative stress. In this study we have investigated the mechanism by which Na+ overload can contribute to the development of cytotoxicity. ATP depletion in isolated hepatocytes exposed to menadione-induced oxidative stress or to KCN was followed by Na+ accumulation, loss of intracellular K+, and cell swelling. Hepatocyte swelling occurred in two phases: a small amplitude swelling (about 15% of the initial size) with preservation of plasma membrane integrity and a terminal large amplitude swelling associated with cell death. Inhibition of Na+ accumulation by the use of a Na+-free medium prevented K+ loss, cell swelling, and cytotoxicity. Conversely, blocking K+ efflux by the addition of BaCl2 did not influence Na+ increase and small amplitude swelling, but greatly stimulated large amplitude swelling and cytotoxicity. Menadione or KCN killing of hepatocytes was also enhanced by inducing cell swelling in an hypotonic medium. However, increasing the osmolarity of the incubation medium did not protect against large amplitude swelling and cytotoxicity, since stimulated Na+ accumulation and K+ efflux. Altogether these results indicate that the impairment of volume regulation in response to the osmotic load caused by Na+ accumulation is critical for the development of cell necrosis induced by mitochondrial inhibition or oxidative stress.

Mechanism of mitochondrial dysfunction and cytotoxicity induced by tropolones in isolated rat hepatocytes

The mechanism of mitochondrial dysfunction and toxicity induced by the tropolones, beta-thujaplicin (4-isopropyl tropolone), tropolone and tropone, has been studied in freshly isolated rat hepatocytes. Incubation of hepatocytes with beta-thujaplicin (1-4 mM) elicited a concentration and time-dependent cell killing. The toxicity was accompanied by losses of cellular ATP, total adenine nucleotides and glutathione, independently of lipid peroxidation and protein thiol oxidation. The beta-thujaplicin-induced cytotoxicity was enhanced by the pretreatment of hepatocyte suspensions with EDTA (4 mM), a hydrophilic chelator, or by incubation in Ca2+ and Mg2+-deficient Krebs-Henseleit buffer. The partition coefficient of beta-thujaplicin, which formed complex with the divalent cations in Krebs-Henseleit buffer, in n-octanol/buffer was increased either in the presence of EDTA or absence of divalent cations. Comparison of toxic effects based on cell viability and adenine nucleotide levels showed that beta-thujaplicin was more toxic than tropolone or tropone in Krebs-Henseleit buffer containing EDTA (4 mM). The addition of beta-thujaplicin to isolated hepatic mitochondria reduced state 3 respiration with NAD+-linked substrate (pyruvate plus malate) and/or with an FAD-linked substrate (succinate plus rotenone), whereas state 3 respiration of ascorbate plus tetramethyl-p-phenylenediamine (cytochrome oxidase-linked respiration) was not significantly affected by beta-thujaplicin. Further, the addition of these tropolones caused a concentration-dependent increase in the rate of state 4 oxygen consumption, indicating an uncoupling effect. These results indicate that beta-thujaplicin- and tropolone-induced cytotoxicity are associated with an acute ATP depletion via mitochondrial dysfunction related to oxidative phosphorylation and that the induction of cytotoxicity is affected by EDTA or divalent cations.

Mechanisms of signal transduction in the stress response of hepatocytes

Adaptation of animals to stress is a unique property of life which allows the survival of the species. The stress response of hepatocytes is a very complex phenomenon, sometimes involving a cascade of events. The general stress signals are elucidated by mobilization of carbohydrate stores and akin to the insulin mediators. Oxidative signals are generated by pesticides, heavy metals, drugs, and alcohol which may or may not be under the purview of peroxisomes. Peroxisomal responses are well-defined involving specific receptors, whereas nonperoxisomal responses may be signaled by calcium, the Ah receptor, or built-in antioxidant systems. The intoxication signals are generally thought to be membrane defects induced by xenobiotics which then lead to highly nonspecific responses of hepatocytes. Detoxication signals, on the other hand, are specific responses of hepatocytes triggering de novo syntheses of detoxifier proteins or enzymes. Evidence reveals the existence of two distinct mechanisms of signal transduction in stressed hepatocytes--one involving the peroxisome and the other the plasma membrane.

Mechanism of p-hydroxybenzoate ester-induced mitochondrial dysfunction and cytotoxicity in isolated rat hepatocytes

The relationship between the metabolism and the cytotoxic effects of the alkyl esters of p-hydroxybenzoic acid (parabens) has been studied in freshly isolated rat hepatocytes. Incubation of hepatocytes with propyl-paraben (0.5 to 2.0 mM) elicited a concentration- and time-dependent cell death that was enhanced when enzymatic hydrolysis of propyl-paraben to p-hydroxybenzoic acid was inhibited by a carboxylesterase inhibitor, diazinon. The cytotoxicity was accompanied by losses of cellular ATP, total adenine nucleotide pools, and reduced glutathione, independently of lipid peroxidation and protein thiol oxidation. In the comparative toxic effects based on cell viability, ATP level, and rhodamine 123 retention, butyl- and isobutyl-parabens were more toxic than propyl- and isopropyl-parabens, and ethyl- and methyl-parabens and p-hydroxybenzoic acid were less toxic than propyl-paraben. The addition of propyl-paraben to isolated hepatic mitochondria reduced state 3 respiration with NAD+-linked substrates (pyruvate plus malate) and/or with an FAD-linked substrate (succinate plus rotenone), whereas state 3 respiration with ascorbate plus tetramethyl-p-phenylenediamine (cytochrome oxidase-linked respiration) was not affected significantly by propyl-paraben. Further, the addition of these parabens caused a concentration-dependent increase in the rate of state 4 oxygen consumption, indicating an uncoupling effect. The rate of state 3 oxygen consumption was inhibited by propyl-paraben, butyl-paraben, and their chain isomers. These results indicate that a) propyl-paraben-induced cytotoxicity is mediated by the parent compound rather than by its metabolite p-hydroxybenzoic acid; b) the toxicity is associated with ATP depletion via impairment of mitochondrial function related to membrane potential and/or oxidative phosphorylation; and c) the toxic potency of parabens to hepatocytes or mitochondria depends on the relative elongation of alkyl side-chains esterified to the carboxyl group of p-hydroxybenzoic acid.

Role of oxidative stress in the selective toxicity of dieldrin in the mouse liver

Dieldrin, an organochlorine insecticide, induces hepatic tumors in mice but not in rats. Although the mechanism(s) responsible for this species specificity is not fully understood, accumulating evidence indicates that oxidative stress may be involved. This study examined the association of dieldrin-induced hepatic DNA synthesis with the modulation of biomarkers of oxidative damage to lipids (malondialdehyde [MDA]) and DNA (8-hydroxy-2-deoxyguanosine [oh8dG]), in male B6C3F1 mice and F344 rats fed dieldrin (0.1, 1.0, or 10 mg/kg diet) for 7, 14, 28, and 90 days. The nonenzymatic components of the antioxidant defense system (ascorbic acid, glutathione, and alpha-tocopherol) were also examined. Increased urinary MDA was observed in mice fed 0.1, 1.0, or 10 mg dieldrin/kg diet for 7, 14, 28, and 90 days; while increased hepatic MDA was seen only after 7 days in mice fed 0.1, 1.0, or 10 mg dieldrin/kg diet and after 14 days in mice fed 10 mg/kg diet. In rats, dieldrin had no effect on either hepatic MDA or urine MDA levels after 7, 14, and 28 days of treatment. A dose-dependent increase in urinary MDA was observed in rats at the 90-day sampling time. The only significant elevation in urinary or hepatic oh8dG content was limited to urinary oh8dG in mice fed 10 mg/kg dieldrin diet for 14 days. Dietary dieldrin produced sustained decreases in hepatic and serum alpha-tocopherol and sustained elevations in hepatic ascorbic acid in both mice and rats. Rats, however, possessed a three- to four-fold higher content of endogenous or basal (control) hepatic alpha-tocopherol; and, even when fed 10 mg dieldrin/kg diet, the levels of hepatic alpha-tocopherol were maintained at higher levels than those of mice fed control diet. In both rats and mice fed dieldrin, transient (14 and 28 days on diet) elevations in hepatic glutathione were observed. These data support the hypothesis that the species specificity of dieldrin-induced hepatotoxicity may be related to dieldrin's ability to induce oxidative stress in the liver of mice, but not in rats. Only in mice fed dieldrin was a temporal association of increases in hepatic MDA content and hepatic DNA synthesis seen, suggesting that oxidative damage (shown by increased lipid peroxidation) may be involved in early events in dieldrin-induced hepatocarcinogenesis. Rats may be protected from dieldrin-induced oxidative stress by a more effective antioxidant defense system, characterized by higher basal levels of hepatic alpha-tocopherol and ascorbic acid than that seen in the mouse.

Thermic transition and glycolytic capacity as critical events in the survival of rat liver slices after overnight cold hypoxic preservation

Cellular survival and hypoxia-reoxygenation injury in overnight cold-preserved liver slices (+/-20 h at 4 degrees C) were investigated. Increased cell death after overnight cold hypoxia depended more on temperature than on the reoxygenation process itself. Fructose (at 50 mM) added before the onset of hypoxia improved survival at the end of 20 h of cold hypoxia over Krebs- or glucose-treated slices. Such a protective effect by fructose was also seen during the normothermic (37 degrees C) reoxygenation of previously cold hypoxic-preserved slices, but only in the absence and not in the presence of tert-butyl hydroperoxide, a model compound widely used to induce an oxidative stress. The protection by fructose was equivalent to that observed when liver slices were incubated in the University of Wisconsin solution (UW). Finally, the morphological study of haematoxylin and eosin (H & E)-stained slices has shown cytoplasmic vacuoles during the reoxygenation step, which were more pronounced in UW-treated than in fructose-treated slices.

Cytotoxic effects of 2,6-di-tert-butyl-4-methylphenyl N-methylcarbamate (terbutol) herbicide on hepatocytes and mitochondria isolated from male rats

The cytotoxic effects of 2,6-di-tert-butyl-4-methylphenyl N-methylcarbamate (terbutol) and its major metabolites were investigated in freshly isolated rat hepatocytes. Terbutol and its metabolite, especially 2,6-di-tert-butyl-4-methylphenyl carbamate (N-demethylterbutol), at a concentration of 1.0 mM resulted in a time dependent cell killing accompanied by losses of intracellular ATP, protein thiols, and glutathione (GSH) and the accumulation of oxidized GSH. Supplementation of the hepatocyte suspension with 5 mM N-acetylcysteine, a precursor of intracellular GSH, inhibited the cytotoxicity of N-demethylterbutol. In mitochondria isolated from rat liver, terbutol and its metabolites impaired respiration related to oxidative phosphorylation and the potency of their toxicity is associated with impairment of mitochondrial respiration. These results indicate that N-demethylterbutol is the most cytotoxic followed by terbutol and other metabolites, and that both the mitochondrial respiratory system and protein thiols are important targets for these compounds.

Protective effect of fructose on survival and metabolic capacities of hepatocytes kept overnight under cold hypoxia before normothermic reoxygenation

The protective effect of fructose with regard to hypoxia-induced cell injury in overnight cold preserved hepatocytes (20 hr at 4 degrees C) was investigated. The addition of fructose (at 10 and 20 mM) resulted in an improved survival of hepatocytes during their normothermic (37 degrees C) reoxygenation, irrespective of the time of fructose addition before the onset of hypoxia (i.e. 10, 20 or 30 min). Such a protective effect was even higher than that observed when hepatocytes were incubated in the University of Wisconsin solution (UW). Moreover, neither Desferal (an iron chelator) nor adenosine (an ATP precursor), nor other carbohydrates (glucose, galactose and the antioxidant mannitol) were able to protect cells against such an hypoxia-mediated injury. The intracellular ATP content was lower in both adenosine- and fructose-treated hepatocytes than in control untreated cells. However, the cellular metabolic capacities such as protein synthesis and gluconeogenesis from lactate recovered faster during reoxygenation of previously hypoxic fructose-treated cells compared with both control and adenosine-treated cells.

Cytosolic Ca2+ and protein kinase Calpha couple cellular metabolism to membrane K+ permeability in a human biliary cell line

Cholangiocytes represent an important target of injury during the ischemia and metabolic stress that accompanies liver preservation. Since K+ efflux serves to minimize injury during ATP depletion in certain other cell types, the purpose of these studies was to evaluate the effects of ATP depletion on plasma membrane K+ permeability of Mz-ChA-1 cells, a model human biliary cell line. Cells were exposed to dinitrophenol (50 microM) and 2-deoxyglucose (10 mM) as the standard model of metabolic injury. Whole-cell and single K+ channel currents were measured using patch clamp techniques; and intracellular [Ca2+] ([Ca2+]i) was estimated by calcium green-1 fluorescence. Metabolic stress increased [Ca2+]i, and stimulated translocation of the alpha isoform of protein kinase C (PKCalpha) from cytosolic to particulate cell fractions. The same maneuver increased membrane K+ permeability 40-70-fold as detected by (a) activation of K+selective whole cell currents of 2,176+/-218 pA (n = 34), and (b) opening of apamin-sensitive K+ channels with a unitary conductance of 17.0+/-0.2 pS. PKCalpha translocation and channel opening appear to be related since stress-induced K+ efflux is inhibited by chelation of cytosolic Ca2+, exposure to the PKC inhibitor chelerythrine (25 microM) and downregulation of PKC by phorbol esters. Moreover, K+ currents were activated by intracellular perfusion with recombinant PKCalpha in the absence of metabolic inhibitors. These findings indicate that in biliary cells apamin-sensitive K+ channels are functionally coupled to cell metabolism and suggest that cytosolic Ca2+ and PKCalpha are selectively involved in the response.

Relationship between mitochondrial dysfunction and toxicity of propyl gallate in isolated rat hepatocytes

The relationship between cytotoxicity and mitochondrial dysfunction caused by propyl gallate (PG) has been studied in hepatocytes freshly prepared from fasted rats. Hepatocytes isolated from fasted (18 h) rats were significantly more susceptible to the toxicity of PG than hepatocytes from fed rats. The addition of fructose (15 mM), an alternative carbohydrate source, to hepatocyte suspensions resulted in the prevention of PG (1 mM)-induced cell killing accompanied by decrease in intracellular ATP loss during a 3 h-incubation period. Despite this, fructose did not completely prevent an abrupt loss of intracellular glutathione caused by PG, but effectively inhibited the loss of protein thiol levels. Fructose elicited a concentration (0.5-20mM)-dependent protection against the cytotoxicity of 1.5 mM PG. The incubation of hepatocytes with sodium azide (4 mM), an inhibitor of oxidative phosphorylation, enhanced the toxicity induced by PG (1 mM), but coincubation with fructose delayed the onset of toxicity. Neither azide alone nor fructose plus azide did affect the cell viability during the incubation period. Furthermore, the addition of 2 mM salicylamide, nontoxic to hepatocytes during the incubation period, enhanced PG (1 mM)-induced cytotoxicity and decreased the loss of free PG. These results indicate that the onset of cytotoxicity caused by PG may depend on the intracellular energy status and that mitochondria are critical target for the compound. In addition, the toxicity caused by the inhibition of mitochondrial ATP synthesis is related to the concentration of PG remaining in cell suspensions.

High energy phosphates and direct calorimetry as predictive parameters for metabolic recovery of the rat liver following ischemia

BACKGROUND AND METHODS

Alteration of the hepatocellular function following ischemic damage may play a crucial role in the limited recovery after reperfusion. In spite of numerous efforts, finding a simple technique for predicting recovery of the liver after ischemic damage is still an unresolved problem. During ischemic storage of isolated rat livers at 25 degrees C tissue concentrations of high energy phosphates and lactate were determined photometrically and interstitial pH was measured by glass electrodes. In comparison, the metabolic rate was measured continuously by direct calorimetry. In a second series of experiments these results were compared with functional recovery after ischemia and reperfusion. Following ischemic storage at 25 degrees C for 60, 120 or 240 min, the isolated livers were reperfused for 30 min in a non-recirculating system with a constant flow rate. During reperfusion functional recovery, as assessed by oxygen consumption and bile flow, was determined. At the end of reperfusion tissue samples were taken for biochemical analysis of adenine nucleotides. Furthermore, morphologic integrity was determined by electron microscopy.

RESULTS

Whereas the ATP concentration drops within 60 min of ischemia to 6.9% of the control value without further significant change, the continuously measured metabolic rate as assessed by direct calorimetry decreases in an exponential manner. Accordingly, a better correlation of hepatocellular secretory function and calorimetrically measured heat output (r2 = 0.85; P < 0.001) was observed than with high energy phosphates (r2 = 0.56; P < 0.001).

CONCLUSIONS

These data suggest that if the metabolism of the ischemic rat liver falls below a critical level, recovery is incomplete or impossible. Therefore, assessment of the global metabolic rate by direct calorimetry seems not only to be a very good predictor of recovery after ischemic damage but also a good tool in the laboratory for studies concerning the sequelae of ischemic metabolism and for improvement of tissue protection.

Metabolic stress opens K+ channels in hepatoma cells through a Ca2+- and protein kinase calpha-dependent mechanism

These studies of a model liver cell line evaluate the mechanisms responsible for regulated release of K+ ions during metabolic stress. Metabolic inhibition of HTC hepatoma cells by exposure to 2, 4-dinitrophenol (50 microM) and 2-deoxy-D-glucose (10 mM) stimulated outward currents carried by K+ of 974 +/- 75 pA at 0 mV (n = 20, p < 0.001). Currents were inhibited by chelation of intracellular Ca2+ or exposure to apamin (50 nM), an inhibitor of SKCa channels. In cell-attached recordings from intact cells, removal of metabolic substrates (25/28 cells) or exposure to metabolic inhibitors (32/40 cells) opened K+-selective channels with a conductance of 6.5 +/- 0. 2 pS. Channels had an open probability of 0.31 +/- 0.08 and opened in bursts averaging 3.55 +/- 0.27 ms in duration (n = 6). Metabolic stress was associated with rapid translocation of the alpha isoform of protein kinase C (PKCalpha) from cytosol to membrane; and down-regulation of PKCalpha by phorbol esters or exposure to the PKC inhibitor chelerythrine (10 microM) each inhibited currents. Moreover, intracellular perfusion with purified PKCalpha activated currents in a Ca2+- and concentration-dependent manner. These findings indicate that metabolic stress leads to opening of apamin-sensitive SKCa channels in hepatoma cells through a Ca2+- and PKC-dependent mechanism and suggest that PKCalpha may be selectively involved in the response. This mechanism functionally couples the metabolic state of cells to membrane K+ permeability and represents a potential target for modification of liver injury associated with ischemia and preservation.

Modulating hypoxia-induced hepatocyte injury by affecting intracellular redox state

Hypoxia-induced hepatocyte injury results not only from ATP depletion but also from reductive stress and oxygen activation. Thus the NADH/NAD+ ratio was markedly increased in isolated hepatocytes maintained under 95% N2/5% CO2 in Krebs-Henseleit buffer well before plasma membrane disruption occurred. Glycolytic nutrients fructose, dihydroxyacetone or glyceraldehyde prevented cytotoxicity, restored the NADH/NAD+ ratio, and prevented complete ATP depletion. However, the NADH generating nutrients sorbitol, xylitol, glycerol and beta-hydroxybutyrate enhanced hypoxic cytotoxicity even though ATP depletion was not affected. On the other hand, NADH oxidising metabolic intermediates oxaloacetate or acetoacetate prevented hypoxic cytotoxicity but did not affect ATP depletion. Restoring the cellular NADH/NAD+ ratio with the artificial electron acceptors dichlorophenolindophenol and Methylene blue also prevented hypoxic injury and partly restored ATP levels. Ethanol which further increased the cellular NADH/NAD+ ratio increased by hypoxia also markedly increased toxicity whereas acetaldehyde which restored the normal cellular NADH/NAD+ ratio, prevented toxicity even though hypoxia induced ATP depletion was little affected by ethanol or acetaldehyde. The viability of hypoxic hepatocytes is therefore more dependent on the maintenance of normal redox homeostasis than ATP levels. GSH may buffer these redox changes as hypoxia caused cell injury much sooner with GSH depleted hepatocytes. Hypoxia also caused an intracellular release of free iron and cytotoxicity was prevented by desferoxamine. Furthermore, increasing the cellular NADH/NAD+ ratio markedly increased the intracellular release of iron. Hypoxia-induced hepatocyte injury was also prevented by oxypurinol, a xanthine oxidase inhibitor. Polyphenolic antioxidants or the superoxide dismutase mimic, TEMPO partly prevented cytotoxicity suggesting that reactive oxygen species contributed to the cytotoxicity. The above results suggests that hypoxia induced hepatocyte injury results from sustained reductive stress and oxygen activation.

Cytotoxic effects of postharvest fungicides, ortho-phenylphenol, thiabendazole and imazalil, on isolated rat hepatocytes

The cytotoxic effects of ortho-phenylphenol (OPP), imazalil (IMZ) and thiabendazole (TBZ) on isolated rat hepatocytes were investigated. Addition of IMZ and OPP to hepatocyte suspensions at a concentration of 0.75 mM resulted in acute cell death, accompanied by depletion of intracellular levels of glutathione and protein thiols. Both compounds rapidly depleted cellular ATP which consistently preceded the cell death. In addition, the cell death caused by IMZ was accompanied by the accumulation of intracellular malondialdehyde, indicating initiation of lipid peroxidation. During a 3-hr incubation period, TBZ did not affect these parameters. In mitochondria isolated from rat liver, IMZ and OPP impaired respiration related to oxidative phosphorylation. Based on these results, the order of toxic potency is IMZ > OPP > TBZ.

Effect of hypoxia and reoxygenation on the formation and release of reactive oxygen species by porcine pulmonary artery endothelial cells

Endothelial cells are critical targets in both hypoxia- and reoxygenation-mediated lung injury. Reactive O2 species (ROS) have been implicated in the pathogenesis of hypoxic and reoxygenation lung injury, and xanthine dehydrogenase/oxidase (XDH/XO) is a major generator of the ROS. Porcine pulmonary artery endothelial cells (PAEC) have no detectable XDH/XO. This study was undertaken to examine 1) ROS production by hypoxic porcine PAEC and their mitochondria and 2) ROS production and injury in reoxygenated PAEC lacking XDH/XO activity. Intracellular H2O2 generation and extracellular H2O2 and O2 divided release were measured after exposure to normoxia (room air-5% CO2), hypoxia (0% O2-95% N-5% CO2), or hypoxia followed by normoxia or hyperoxia (95% O2-5% CO2). Exposure to hypoxia results in significant reductions in intracellular H2O2 formation and extracellular release of H2O2 and O2 by PAEC and mitochondria. The reductions occur with as little as a 2 h exposure and progress with continued exposure. During reoxygenation, cytotoxicity was not observed, and the production of ROS by PAEC and their mitochondria never exceeded levels observed in normoxic cells. The absence of XDH/XO may prevent porcine PAEC from developing injury and increased ROS production during reoxygenation.

Cellular acidification occurs during anoxia in cultured, but not in freshly isolated, rabbit proximal tubular cells

In a variety of cells it has been shown that acidosis is protective against anoxic injury. We have demonstrated previously that proximal tubule (PT) cells in primary culture were more resistant to anoxia-induced cell injury than were freshly isolated cells. Therefore, we asked the question of whether a difference in cellular acidification during anoxia could explain this difference in susceptibility to anoxia. To answer this question, intracellular pH (pHi) was measured during anoxic incubation of PT cells in culture and those that were freshly isolated. PT cells were incubated in an anoxic chamber at 37 degrees C after loading with 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl ester (BCECF-AM) or fura-2 acetoxymethyl ester (fura-2-AM). pHi and cytosolic free Ca2+ ([Ca2+]i) were measured by digital imaging fluorescence microscopy. During anoxia, pHi in cultured PT cells decreased from 7.3 +/- 0.1 to 6.8 +/- 0.1, whereas pHi in freshly isolated cells did not decrease significantly. In addition, the intrinsic buffering capacities (beta i) in cultured and freshly isolated PT cells were determined and turned out to be the same at a pHi greater than or equal to 7.3. Below pHi 7.3, beta i increased several fold in freshly isolated PT cells, and rose to significantly higher levels than in cultured PT cells. During 1 h of anoxia, cell viability of freshly isolated PT cells decreased significantly to 54% +/- 2% (P < 0.05), while no loss in viability was observed in cultured PT cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Desferal prevents against cell lysis induced by hydrogen peroxide to hypoxic hepatocytes: a role for free iron in hypoxia-mediated cellular injury

Isolated hepatocytes incubated under hypoxic conditions were more sensitive to H2O2-mediated injury as compared to cells kept under aerobic conditions, but only for the highest H2O2 concentration tested (8 mM). At lower concentrations (2 and 4 mM) cells were still able to detoxify H2O2 even under hypoxic conditions. Reoxygenation of hypoxic hepatocytes did not result in a cytolytic effect, whereas reoxygenation in the presence of H2O2 resulted in an enhanced cytotoxicity. The duration of previous hypoxia (before H2O2 addition) did not affect the lytic effect induced by H2O2. Enzymatic activities of both catalase and glutathione peroxidase were unchanged over 2 h of incubation under hypoxic conditions. Preincubation of hepatocytes in the presence of Desferal (5 mM) resulted in the abolition of H2O2-mediated lytic effects. A role for free iron, released from intracellular stores and acting on H2O2 to yield reactive oxygen species is discussed.

Cytotoxicity of propyl gallate and related compounds in rat hepatocytes

The cytotoxic effects of propyl gallate (PG), its related gallates and gallic acid have been studied in freshly isolated rat hepatocytes. Addition of PG (0.5-2.0 mM) to hepatocyte suspension elicited concentration-dependent cell death accompanied by losses of intracellular ATP, adenine nucleotide pools, glutathione (GSH) and protein thiols. The rapid loss of intracellular ATP preceded the onset of cell death caused by PG. In the comparative toxic effects of PG and related gallates at concentration of 1 mM, octyl gallate (OG), dodecyl gallate (DG) and butyl gallate (BG) elicited an abrupt depletion of ATP, followed by an acute cell death. These gallates were more toxic than PG; the toxic effects of PG were similar to those of methyl gallate (MG) and ethyl gallate (EG). In mitochondria isolated from rat liver, PG caused a concentration-dependent increase in the rate of state 4 oxygen consumption, indicating an uncoupling effect. The rate of state 3 oxygen consumption was inhibited by OG and DG. According to the respiratory control index, the order of impairment potency to mitochondria was OG > BG, DG > PG > EG, MG > gallic acid. These results indicate that PG and related gallates are toxic to hepatocytes and that the acute cytotoxicity may be due to mitochondrial dysfunction.